Longitude - Page 1

Longitude - Page 2

Running along a line of latitude

Lines of latitude run around the world in an east-west direction. Sailing along a line of latitude was a well established way of navigating at that time. If you are running along a line of latitude, you know you are veering too far south if the sun starts getting higher at noon, and likewise you are heading too far north if the sun starts getting lower at local noon. Of course the height of the sun changes throughout the year, and you need a set of tables that allow you to make the appropriate calculation. But to put things at their simplest, if you lived in Bristol in the UK, and you knew the height of the sun above the horizon on any given day of the year, it would be easy to set sail going west, making sure on any particular day of the year, the sun at local noon aboard your ship was kept the same as it would have been back in Bristol. If the sun started getting too high or low, it would show you were drifting off course, and you would need to correct that. Using this method, and the only navigation instrument you would need would be something like a cross staff, you would run down a line of latitude that was the same as that of Bristol, and you would reach landfall in the north of Newfoundland. The only thing you would not know during your voyage was your longitude, so you would not know how far you had gone, or indeed how far you still had to go.

Back aboard the Centurion…

The Centurion sailed north, until they were at the correct latitude. That was the easy bit. But, as they had no way of working out their longitude, they had no way of knowing if they were east of west of the island. If they were west of the island, they needed to sail east, otherwise they would just head off into the Pacific. But if they were east of the island, they needed to sail west, otherwise they would hit the mainland, and that ran the danger of warning the Spanish of their arrival. And time was running out.

Their first guess, to sail west, was actually right, although they didn't know it at the time. But having sailed west until they were in fact very close to the island, they decided they had been wrong. They turned the ship around, and sailed east. But when they saw the mainland ahead, they realised to their dismay that they were now sailing in the wrong direction, and so they had to turn the ship around yet again, to sail west. Because of not knowing their longitude, they had to sail an extra thousand miles, another hundred men died, and they reached the island with barely enough crew left to sail the ship.

Introducing Longitude…

We all know that if we are having lunch in London, people in New York are having breakfast - there is a five hour time difference between London and New York. But this works in just the same way on a smaller scale. The following story may seem like a detour, but bear with us - it all connects back to longitude!

150 years ago, towns in the UK all worked to local time - noon was the moment when the sun was highest in the sky. Across the UK there is a time difference of half an hour. So at that time churches on the east coast would strike noon half an hour earlier than churches on the west coast. This had been the case for hundreds of years, and had not been a problem for anyone. Who cared if the clocks in a distant town were a few minutes different?

However just before 1800, this concept of local time started to matter. The Royal Mail started a network of coaches that connected different towns around the country, and these coaches needed to run to a strict timetable. One story says that there was a penny fine for every minute the coach was late! The guard was issued with a special watch, in a leather pouch, which ensured that the coach ran to time. But there was a problem.